1. Field of the Invention
The present invention relates to a fundus camera which is capable of photographing of a fundus image, and more particularly to a fundus camera which is suitable for photographing of a fundus image of an eye of an examinee, particularly a man who lies on his back, babies and little children and animals and so on.
2. Description of Related Art
For an illumination optical system of so called a hand type fundus camera, conventionally, the following two kinds of methods have been proposed.
One is a separate illumination method wherein an illumination system and a photographing system are separately constructed, the illumination system having a small prism disposed slightly under a photographing optical path between an objective lens of the photographing system and an eye of an examinee, thereby illuminating the examinee's eye obliquely from under the eye.
Another is a coaxial illumination method wherein a semi-transparent mirror is disposed between an objective lens of a photographing system and an examinee's eye, so that a light beam of the illumination system and same of the photographing system are transmitted along optical paths being coaxial.
The former separate illumination method has the advantage of miniatuarizing an illumination system because the optical system thereof is simple and also may get the sufficient reflection light quantity from a fundus of the examinee's eye. On the other hand, preventing the prism of the illumination system from eclipsing a light beam of the photographing system and supplying illumination light to the fundus of the eye over the photographing visual field cause a demerit of a working distance coming to be very short. If the working distance between the examinee's eye and the apparatus comes to be short, it gives the examinee a dangerous feeling and an oppressive feeling, so that the examinee is apt to frequently blink his eyes and to move nervously his eyeball. Consequently, such motions often cause a bad influence on photographing of the fundus image of the eye.
Additionally, as the illumination system illuminates the examinee's eye from obliquely under the eye, upper and lower parts of a photographed picture are liable to often lack uniformity of illumination. When the pupil of the examinee's eye having a small diameter, illumination luminous flux is difficult to direct inside the eye, so that such an illumination system is not suitable for a photographing with no use of mydriatica, which utilizing natural mydriasis.
The latter illumination system has a merit of a longer working distance compared with the former separate illumination system, as well as a merit of not causing lack of uniformity of illumination. However, in this illumination system, large loss of light quantity is caused by the semi-transparent mirror, which is then reduced to about a quarter of the light quantity. As a result, if the illumination system is used in fundus cameras which need sufficient light quantity for photographing, then a light source of illumination light must be remodeled to supply large quantity of light and to be large-sized.